Image pickup lens and design method thereof

ABSTRACT

Provided in the present invention are an image pickup lens and a design method thereof which can achieve miniaturization of the system and capable of remarkably improving the optical characteristic with a simple structure. By forming a second face on the image pickup surface side of a lens body into a Fresnel face and unifying a diffraction element with at least a first face on the object face side or the second face on the image pickup surface side of the lens body, it becomes possible to correct the Petzval sum and reduce the curvature of the field so that an excellent image plane can be obtained. Also, due to the color dispersion characteristic of the diffraction element, chromatic aberration can be well corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image pickup lens and a design method thereof and particularly, to an image pickup lens comprising one lens which can be reduced in size and weight while achieving a wider angle of view to be used for an image pickup device (e.g., a CCD camera) utilizing an image pickup element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone and the like, and a design method thereof.

[0003] 2. Description of the Related Art

[0004] Recently, there is a remarkable development in the multimedia industry. For example, there has been an increasing demand for a camera (e.g., a CCD camera) utilizing an image pickup element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone and the like. Such CCD camera needs to be mounted on a limited space. Thus, it is desirable that the camera be small in size and light in weight.

[0005] Accordingly, an image pickup lens used for such CCD camera is also required to be small and lightweight as well.

[0006] Conventionally, the so-called one-lens system using a single lens is used as such image pickup lens.

[0007]FIG. 1 shows a conventional image pickup lens with a one-lens system. The one-lens system comprises a lens body 10 having a positive power and a diaphragm 11 is disposed on an object side of the lens body 10. Further, on the image plane side of the lens body 10, a first cover glass 12, a second cover glass 13 and an image pickup surface 14 as a light receiving surface for an image pickup element such as a CCD, a CMOS or the like are disposed in order. Each lens face of the lens body is referred to be the first face and the second face, respectively, in order from the object side.

[0008] The image pickup lens is set under the following condition:

f=4. 10 mm, F No=2.2, 2ω=60.0° Petzval sum=0.681

[0009] where, f denotes the focal length (mm) of the whole system, F No denotes F number, and 2ω denotes the maximum angle of view. Also, r denotes the radius of curvature (mm) of the lens and the like, d denotes the distance (mm) between each optical face, nd denotes the refractive index of an optical material (medium) present between with the next optical face, and νd denotes the Abbe constant.

[0010] Provided the optical axis direction is taken as a Z-axis, the direction perpendicular to the optical axis is taken as an X-axis, and the traveling direction of light is defined to be positive, the shape of the aspherical face of the lens is expressed by a following expression (Eq1): $Z = {\frac{\frac{x^{2}}{r}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{x^{2}}{r^{2}}}}} + {a_{4\quad}x^{4}} + {a_{6\quad}x^{6}}}$

[0011] where,

[0012] each of k, a₄, a₆ is an aspherical factor. Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.629 (2) First face of 49.774 2.871 1.584 31.0 lens body (3) Second face of −2.477 1.000 lens body (4) First face of 0.000 0.750 1.517 64.0 first cover glass (5) Second face of 0.000 0.250 first cover glass (6) First face of 0.000 1.200 1.517 64.0 second cover glass (7) Second face of 0.000 1.518 second cover glass (8) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2  1.468005e+003 −1.312730e−002 −7.674194e−003 3 −1.315709e+000 −4.263500e−002 −1.495094e−003

[0013] However, in the one-lens image pickup lens of the related art, the Petzval sum and the curvature of field are large so that an excellent image plane cannot be obtained. Also, each aberration such as the longitudinal chromatic aberration, the chromatic aberration of magnification, the spherical aberration, the astigmatism is large so that each aberration cannot be corrected appropriately. Therefore, it is not possible to obtain an excellent optical characteristic.

[0014]FIG. 2 shows the chromatic aberration of magnification of ray paths A, B, and C in regards to the image pickup lens shown in FIG. 1. It shows that the chromatic aberration of magnification is large in the related art and it becomes large especially in the vicinity of the image pickup lens, thereby resulting in deterioration of the lens property. FIG. 3 shows the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens. It shows that each of the spherical aberration, the astigmatism, and the distortion aberration is large and, particularly, the spherical aberration and the stigmatism are large. Thus, it is clear that a sufficient optical characteristic cannot be obtained.

[0015] In view of this respect, there is a technique of the related art as disclosed in Japanese Patent Application Laid-open No. 10-73760 in which a diffraction element is unified with a refractive lens so as to correct the chromatic aberration of the image pickup lens by the diffraction element.

[0016] However, with this lens, the Petzval sum cannot be corrected thereby causing a large curvature of field. The reason is that there is no contribution by the diffraction element imposed onto the Petzval sum so that it is determined only by the property of a refractive lens.

SUMMARY OF THE INVENTION

[0017] The invention has been designed to overcome the foregoing problems. It is an object of the present invention to provide an image pickup lens and a design method thereof, which can achieve minimization of the system and a remarkable improvement in the optical characteristic with a simple structure.

[0018] In order to achieve the aforementioned objects, an image pickup lens of the present invention comprises a lens body having a second face on an image pickup surface side of the lens body being formed into a Fresnel face and a diffraction element being unified with at least either a first face of on an object side or the second face on the image pickup surface side of the lens body.

[0019] In the invention constituted as described, the second face on the image pickup surface side of the lens body is formed into a Fresnel face. Thus, it becomes possible to correct the Petzval sum and reduce the curvature of the field so that an excellent image plane can be obtained. Also, the diffraction element is unified at least with either the first face on the object side or the second face on the image pickup surface side of the lens body. Therefore, due to a color dispersion characteristic of the diffraction element, the chromatic aberration can be well corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.

[0020] Also, the diffraction element may be unified with the second face of the lens body.

[0021] By unifying the diffraction element with the second face of the lens body, chromatic aberration can be well corrected and the optical characteristic of the image pickup lens can be remarkably improved due to a color dispersion characteristic of the diffraction element.

[0022] Further, the first face of the lens body may be formed into an aspheric face and the second face of the lens body may be formed into an aspheric Fresnel face.

[0023] By forming the first face of the lens body into an aspheric face and the second face of the lens body may into an aspheric Fresnel face, each aberration such as the spherical aberration and the like can be appropriately corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.

[0024] Furthermore, in the image pickup lens of the present invention, the lens body may be formed to satisfy a condition represented by a following expression:

1.25<d/f<1.45   (1)

[0025] where,

[0026] d: thickness in the center of the lens body; and

[0027] f: focal length of the lens body.

[0028] By satisfying the expression (1), the distortion aberration amount can be corrected with a comfortable visual sense while well maintaining each aberration except for the distortion aberration. In the expression (1), if the d/f value is larger than 1.45, the distortion aberration becomes large in the positive direction so that the back focus becomes short. If the d/f value is smaller than 1.25, the distortion aberration in the negative direction becomes large so that, even though the distortion aberration can be suppressed, the curvature of field, the longitudinal chromatic aberration, the lateral aberration, the spherical aberration and the like cannot be well corrected.

[0029] Also, in the image pickup lens of the present invention, the lens body may be formed to satisfy a condition represented by a following expression:

0.6<|r ₂ /r ₁|<0.75   (2)

[0030] where,

[0031] r₁: radius of curvature in the center of the first face of the lens body; and

[0032] r₂: radius of curvature in the center of the second face of the lens body.

[0033] By satisfying the expression (2), an ideal lens shape in which each aberration is well balanced can be achieved. In the expression (2), if the |r₂/r₁| value is larger than 0.75 or smaller than 0.6, mainly the spherical aberration becomes worsen resulting in deterioration of resolution.

[0034] Further, in the image pickup lens of the present invention, a diaphragm may be positioned on the object side of the lens body and the lens body may be formed to satisfy a condition represented by a following expression:

0≦ds/f≦0.4   (3)

[0035] where,

[0036] ds: distance between the center of diaphragm and the first face of the lens body; and

[0037] f: focal length of the lens body.

[0038] By satisfying the expression (3), the coma aberration can be corrected while achieving an excellent balance of coma aberration and the distortion aberration. In the expression (3), if the ds/f value is larger than 0.4, the coma aberration cannot be sufficiently corrected and if it is smaller than 0, the diaphragm is to be in the lens body. Thus, it is not desirable since it becomes difficult to manufacture.

[0039] A design method of an image pickup lens according to the present invention is for determining the thickness and the radius of curvature in the center of a lens body having a second face on an image pickup surface side of the lens body being formed into a Fresnel face and a diffraction element being unified with at least either a first face on the object side or the second face on the image pickup surface side of the lens body. The design method comprises the steps of: under the condition that each aberration is maintained to be excellent except for distortion aberration, performing ray tracing simulation beforehand on a lens body group with a different combination of the d/f value (where, d denotes the thickness in the center of a lens body and f denotes the focal length of a lens body) and the |r₂/r₁| value (where, r₁ denotes the radius of curvature in the center of the first face of the lens body and r₂ denotes the radius of curvature in the center of the second face of the lens body); calculating the distortion aberration of the lens bodies so as to obtain dependency of the distortion aberration on the d/f value and the |r₂/r₁| value; selecting, according to the correlation, the combination of the d/f value and the |r₂/r₁| value to be a desired distortion aberration; and determining the thickness and the radius of curvature of the lens body on the basis of the d/f value and the |r₂/r₁| value.

[0040] In the method of the present invention as described, the thickness in the center of the lens body and the radius of the curvature in the center are determined based on the d/f value and |r₂/r₁| value obtained beforehand. Therefore, it becomes possible to design a lens body in which distortion aberration is appropriately corrected.

[0041] Further, in the image pickup lens of the present invention comprises a lens body having at least one face being formed into an aspherical face and at least either the first face on the object side of or the second face on the image pickup surface side of the lens body being formed into a Fresnel face, and the lens body may be formed to satisfy a condition represented by a following expression:

1.17<d/f<1.4   (4)

[0042] where,

[0043] d: thickness in the center of the lens body; and

[0044] f: focal length of the lens body.

[0045] By forming at least one face of the lens body into an aspherical face and at least either the first face on the object side or the second face on the image pickup surface side of the lens body into a Fresnel face, it becomes possible to correct the Petzval sum and reduce the curvature of the field so that an excellent image plane can be obtained. Also, each aberration such as spherical aberration and the like can be well corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.

[0046] Also, by satisfying the expression (4), the distortion aberration amount with a comfortable visual sense can be achieved while well maintaining the aberration except for the distortion aberration. In the expression (4), if the d/f value is larger than 1.4, the distortion aberration becomes large in the positive direction so that the back focus becomes short. If the d/f value is smaller than 1.17, the distortion aberration in the negative direction becomes large so that, even though the distortion aberration can be suppressed, the curvature of field, the longitudinal chromatic aberration, the lateral aberration, the spherical aberration and the like cannot be well corrected.

[0047] Further, in the image pickup lens of the present invention, the lens body may be formed to satisfy a condition represented by a following expression:

0.58<|r ₂ /r ₁|<0.73   (5)

[0048] where,

[0049] r₁: radius of curvature in the center of the first face of the lens body; and

[0050] r₂: radius of curvature in the center of the second face of the lens body.

[0051] By satisfying the expression (5), an ideal lens shape can be achieved in which each aberration is well balanced. In the expression (5), if the |r₂/r₁| value is larger than 0.73 or smaller than 0.58, mainly the spherical aberration becomes worsen resulting in deterioration of resolution.

[0052] Furthermore, in the image pickup lens of the present invention, a diaphragm may be positioned on the object side of the lens body and the lens body may be formed to satisfy a condition represented by a following expression:

0≦ds/f≦0.4   (6)

[0053] where,

[0054] ds: distance between the center of diaphragm and the first face of the lens body; and

[0055] f: focal length of the lens body.

[0056] By satisfying the expression (6), the coma aberration can be corrected while achieving an excellent balance of coma aberration and the distortion aberration. In the expression (6), if the ds/f value is larger than 0.4, the coma aberration cannot be sufficiently corrected and if it is smaller than 0, the diaphragm is to be in the lens body. Thus, it is not desirable since it becomes difficult to manufacture.

[0057] Furthermore, a design method of an image pickup lens according to the present invention is for determining the thickness and the radius of curvature in the center of a lens body having at least the first face on the object side or the second face on the image pickup surface side of the lens body being formed into a Fresnel face. The design method comprises the steps of: under the condition that each aberration is maintained to be excellent except for the distortion aberration, performing ray tracing simulation beforehand on a lens body group with a different combination of d/f value (where, d denotes the thickness in the center of a lens body and f denotes the focal length of a lens body) and |r₂/r₁| value (where, r₁ denotes the radius of curvature in the center of the first face of said lens body and r₂ denotes the radius of curvature in the center of the second face of the lens body); calculating the distortion aberration of the lens bodies so as to obtain dependency of the distortion aberration on the d/f value and the |r₂/r₁| value; selecting, according to the correlation, the combination of the d/f value and the |r₂/r₁| value to be a desired distortion aberration; and determining the thickness and the radius of curvature of the lens body on the basis of the d/f value and the |r₂/r₁| value.

[0058] In the method of the present invention as described, the thickness in the center of the lens body and the radius of the curvature in the center are determined based on the d/f value and |r₂/r₁| value obtained beforehand. Therefore, it becomes possible to design a lens body in which distortion aberration is appropriately corrected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059]FIG. 1 is a schematic illustration showing an image pickup lens of the related art;

[0060]FIG. 2 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 1;

[0061]FIG. 3 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 1;

[0062]FIG. 4 is a schematic illustration showing an embodiment of the image pickup lens according to the present invention;

[0063]FIG. 5 is an enlarged view of a second face of the image pickup lens shown in FIG. 4;

[0064]FIG. 6 is a schematic illustration showing another embodiment of the image pickup lens according to the present invention;

[0065]FIG. 7 is a graph showing the correlation between the distortion aberration and the d/f value in the image pickup lens according to the present invention;

[0066]FIG. 8 is a graph showing the correlation between the distortion aberration and the |r₂/r₁| value in the image pickup lens according to the present invention;

[0067]FIG. 9 is a schematic illustration showing Example 1 of the image pickup lens according to the present invention;

[0068]FIG. 10 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 9;

[0069]FIG. 11 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 9;

[0070]FIG. 12 is a schematic illustration showing Example 2 of the image pickup lens according to the present invention;

[0071]FIG. 13 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 12;

[0072]FIG. 14 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 12;

[0073]FIG. 15 is a schematic illustration showing Example 3 of the image pickup lens according to the present invention;

[0074]FIG. 16 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 15;

[0075]FIG. 17 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 15;

[0076]FIG. 18 is a schematic illustration showing Example 4 of the image pickup lens according to the present invention;

[0077]FIG. 19 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 18;

[0078]FIG. 20 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 18;

[0079]FIG. 21 is a schematic illustration showing Example 5 of the image pickup lens according to the present invention;

[0080]FIG. 22 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 21;

[0081]FIG. 23 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 21;

[0082]FIG. 24 is a schematic illustration showing Example 6 of the image pickup lens according to the present invention;

[0083]FIG. 25 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 24;

[0084]FIG. 26 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 24;

[0085]FIG. 27 is a schematic illustration showing Example 7 of the image pickup lens according to the present invention;

[0086]FIG. 28 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 27;

[0087]FIG. 29 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 27;

[0088]FIG. 30 is a schematic illustration showing Example 8 of the image pickup lens according to the present invention;

[0089]FIG. 31 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 30;

[0090]FIG. 32 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 30;

[0091]FIG. 33 is a schematic illustration showing Example 9 of the image pickup lens according to the present invention;

[0092]FIG. 34 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 33;

[0093]FIG. 35 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 33;

[0094]FIG. 36 is a schematic illustration showing Example 10 of the image pickup lens according to the present invention;

[0095]FIG. 37 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 36;

[0096]FIG. 38 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 36;

[0097]FIG. 39 is a schematic illustration showing Example 11 of the image pickup lens according to the present invention;

[0098]FIG. 40 is an explanatory illustration showing the chromatic aberration of magnification of the image pickup lens shown in FIG. 39;

[0099]FIG. 41 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 39;

[0100]FIG. 42 is a schematic illustration showing still another embodiment of the image pickup lens according to the present invention;

[0101]FIG. 43 is a graph showing the correlation between the distortion aberration and the d/f value in the image pickup lens according to the present invention;

[0102]FIG. 44 is a graph showing the correlation between the distortion aberration and the |r₂/r₁| value in the image pickup lens according to the present invention;

[0103]FIG. 45 is a schematic illustration showing Example 12 of the image pickup lens according to the present invention;

[0104]FIG. 46 is an explanatory illustration showing the lateral aberration of the image pickup lens shown in FIG. 45;

[0105]FIG. 47 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 45;

[0106]FIG. 48 is a schematic illustration showing Example 13 of the image pickup lens according to the present invention;

[0107]FIG. 49 is an explanatory illustration showing the lateral aberration of the image pickup lens shown in FIG. 48;

[0108]FIG. 50 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 48;

[0109]FIG. 51 is a schematic illustration showing Example 14 of the image pickup lens according to the present invention;

[0110]FIG. 52 is an explanatory illustration showing the lateral aberration of the image pickup lens shown in FIG. 51;

[0111]FIG. 53 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 51;

[0112]FIG. 54 is a schematic illustration showing Example 15 of the image pickup lens according to the present invention;

[0113]FIG. 55 is an explanatory illustration showing the lateral aberration of the image pickup lens shown in FIG. 54;

[0114]FIG. 56 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 54;

[0115]FIG. 57 is a schematic illustration showing Example 16 of the image pickup lens according to the present invention;

[0116]FIG. 58 is an explanatory illustration showing the lateral aberration of the image pickup lens shown in FIG. 57; and

[0117]FIG. 59 is an explanatory illustration showing the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens shown in FIG. 57.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0118] Now, one of a basic embodiment according to the present invention will be described by referring to FIG. 4 to FIG. 8.

[0119]FIG. 4 shows a basic structure of an image pickup lens according to the present invention. The image pickup lens comprises a lens body la made of a resin such as amorphous polyolefin or the like. In this basic embodiment, as shown in FIG. 5, a second face on the image pickup surface side of the lens body 1 a is formed into an aspherical Fresnel face 2 a and, at the same time, diffraction grooves are formed on the second face and a diffraction element 3 a is unified therewith. Further, a first face of the lens body la is formed into a spherical face.

[0120] The diffraction element 3 a may be formed on the first face on an object side of the lens body 1 a or on both the first and second faces of the lens body la.

[0121] A diaphragm 4 a is disposed on the object side of the lens body 1 a. Also, a cover glass 5 a and an image pickup surface 6 a as a light receiving surface of an image pickup element such as a CCD, a CMOS or the like are disposed, respectively, on the second face side of the lens body 1 a.

[0122] In the embodiment, the second face on the image pickup surface 6 a side of the lens body 1 a is formed into the Fresnel face 2 a so that Petzval sum can be corrected. Thus, the curvature of field can be reduced and an excellent image plane can be obtained. Also, the diffraction element 3 a is unified with at least either the first or the second face of the lens body la so that, due to the color dispersion characteristic of the diffraction element 3 a, chromatic aberration can be well corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.

[0123]FIG. 6 shows another basic embodiment of the present invention. In this embodiment, as in the same manner as the above-described embodiment, the image pickup lens comprises a lens body 1 a and a second face on the image pickup surface side of the lens body 1 a is formed into an aspherical Fresnel face 2 a. At the same time, diffraction grooves are formed on the second face and a diffraction element 3 a is unified therewith. Further, a first face of the lens body 1 a is formed into a spherical face.

[0124] A diaphragm 4 a is disposed on the object side of the lens body 1 a. Also, a cover glass 6 a and an image pickup surface 6 a as a light receiving surface of an image pickup element such as a CCD, a CMOS or the like are disposed, respectively, on the second face side of the lens body 1 a.

[0125] Further, in the embodiment, the lens body 1 a is to satisfy the condition represented by the following expressions:

1.25<d/f<1.45   (1)

0.6<|r ₂ /r ₁|<0.75   (2)

0≦ds/f≦0.4   (3)

[0126] where,

[0127] d denotes the thickness in the center of the lens body, f denotes the focal length of the lens body, r₁ denotes the radius of curvature in the center of the first face of the lens body, r₂ denotes the radius of curvature in the center of the second face of the lens body, and ds denotes the distance between the center of the diaphragm and the first face of the lens body.

[0128] In the present embodiment, when determining the above-described expression (1) at the time of designing the lens body, first, ray tracing simulation is performed beforehand on a group of lens bodies with different d/f values under the condition where each aberration except for the distortion aberration is maintained to be excellent. Then, dependency of the distortion aberration on the d/f value is obtained by calculating the distortion aberration of the lens bodies. The d/f value as a desired distortion aberration is selected on the basis of the correlation obtained thereby so as to determine the thickness in the center of the lens body based on the selected d/f value.

[0129]FIG. 7 shows the dependency of the distortion aberration on the d/f value. In general, the amount of distortion aberration which is visually comfortable is about −4% to 2% in the field of the present invention. Thus, as shown in FIG. 7, the appropriate d/f value within the range of appropriate distortion aberration amount is set within the large frame shown in the figure. The thickness in the center of the lens body is to be determined on the basis of the d/f value within the range.

[0130] More preferably, the distortion aberration amount may fall within the range of −3% to 2%. Under the condition, the appropriate d/f value within the range of the appropriate distortion aberration amount is set within the small frame shown in FIG. 7. This can be represented by a following expression:

1.28<d/f<1.43   (3a)

[0131] Also, in the same manner, when determining the expression (2), ray tracing simulation is performed beforehand on a group of lens bodies with different |r₂/r₁| values under the condition where each aberration except for the distortion aberration is maintained to be excellent. Then, dependency of the distortion aberration on the |r₂/r₁| value is obtained by calculating the distortion aberration of the lens bodies. The |r₂/r₁| value as a desired distortion aberration is selected on the basis of the correlation obtained thereby so as to determine the radius of curvature in the center of the lens body based on the selected |r₂/r₁| value.

[0132]FIG. 8 shows the dependency of the distortion aberration on the |r₂/r₁| value. The appropriate |r₂/r₁| value within the range of appropriate distortion aberration amount for achieving −4% to 2% distortion aberration falls within the large frame show in FIG. 8. The radius of curvature in the center of the lens body is to be determined on the basis of the |r₂/r₁| value within the range.

[0133] More preferably, the distortion aberration amount may fall within the range of −3% to 2%. Under the condition, the appropriate |r₂/r₁| value within the range of appropriate distortion aberration amount is set within the small frame shown in FIG. 8. This can be represented by a following expression:

0.62<|r ₂ /r ₁|<0.72   (3a)

[0134] The expression (1) is a condition for correcting the distortion aberration amount with a comfortable visual sense while well maintaining the aberration except for the distortion aberration. In the expression (1), if the d/f value is larger than 1.45, the distortion aberration becomes large in the positive direction so that the back focus becomes short. If the d/f value is smaller than 1.25, the distortion aberration in the negative direction becomes large. Thus, even though the distortion aberration can be suppressed, the curvature of field, the longitudinal chromatic aberration, the lateral aberration, the spherical aberration and the like cannot be well corrected.

[0135] Further, the expression (2) is a condition for achieving an ideal lens shape in which each aberration is well balanced. In the expression (2), if the |r₂/r₁| value is larger than 0.75 or smaller than 0.6, mainly the spherical aberration becomes worsen resulting in deterioration of resolution.

[0136] Furthermore, the expression (3) is a condition for correcting the coma aberration while achieving an excellent balance of coma aberration and the distortion aberration. In the expression (3), if the ds/f value is larger than 0.4, the coma aberration cannot be sufficiently corrected and if it is smaller than 0, the diaphragm is to be in the lens body. Thus, it is not desirable since it becomes difficult to manufacture.

[0137] In the present embodiment, by satisfying the expressions (1) to (3), the distortion aberration can be well corrected compared to the case of the previously described embodiment shown in FIG. 4.

EXAMPLES

[0138] Now, Examples of the present invention will be described by referring to FIG. 9 to FIG. 41.

[0139] In the Examples, f denotes the focal length (mm) of the whole system, F No denotes F number, and 2ω denotes the maximum angle of view. Further, r denotes the radius of curvature (mm) in the center of the optical surface, d denotes the distance (mm) between each optical surface on the optical axis, nd denotes the refractive index of an optical material (medium) present between with the next optical surface, and σd denotes the Abbe constant.

[0140] Provided the optical axis direction is taken as a Z-axis, the direction perpendicular to the optical axis is taken as an X-axis, and the traveling direction of light is defined to be positive, the shape of the aspheric face of the lens is represented by a following expression (Eq 2): $Z = {\frac{\frac{x^{2}}{r}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{x^{2}}{r^{2}}}}} + {a_{4\quad}\quad x^{4}} + {a_{6\quad}\quad x^{6}} + {a_{8}\quad x^{8}} + {a_{10}\quad x^{10}}}$

[0141] where,

[0142] each of k, a₄, a₆, a₈ and a₁₀ is an aspherical factor.

[0143] Furthermore, in the present basic embodiment, optical design of the diffraction element is achieved using high diffractive index method according to a monograph “Mathematical equivalence between a holographic optical element and an ultra-high index lens (Optical Society of America, Vol. 69, No. Mar. 3, 1979)” by William C. Sweatt. The diffraction element is approximately provided by a high refractive index thin film with the refractive index nd of a d line (587.56 nm)=894.498 and the Abbe constant σd=−3.45, thereby to design an achromatic lens using a primary diffraction light.

Example 1

[0144]FIG. 9 shows Example 1 of the present invention. In Example 1, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the first embodiment shown in FIG. 4.

[0145] The image pickup lens of Example 1 was set under the following condition: f = 3.5 mm, F No = 2.2, 2ω = 71.9, Petzval sum = 0.33 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.0433 (2) First face of 4.704 3.8538 1.518 56.0 lens body (3) Second face of −2.359 0.0000 894.498 −3.45 lens body (4) High refractive −2.359 1.0000 index layer (5) First face of 0.000 1.0000 1.5163 64.0 cover glass (6) Second face of 0.000 0.8602 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2  2.156864e+01 −4.111183e−02 2.443172e−02 3 −9.965983e−01  8.726602e−04 2.241136e−04 4 −9.947408e−01  8.902002e−04 2.238210e−04 Face a₈ a₁₀ 2 −5.527421e−02 7.460217e−04 3  1.966780e−06 2.448387e−07 4  2.033151e−06 2.443486e−07

[0146]FIG. 10 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 1 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0147] Further, FIG. 11 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 1. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 2

[0148]FIG. 12 shows Example 2 of the present invention. In Example 2, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the first basic embodiment shown in FIG. 4. Further, it was a wide-angle lens in which the focal length was shortened and the angle of view was widened.

[0149] The image pickup lens of Example 2 was set under the following condition: f = 3.2 mm, F No = 2.2, 2ω = 77.8, Petzval sum = 0.36 Radius r Abbe Face of Curvature Distance d Refractive constant νd (1) Diaphragm 0.000 0.0248 (2) First face of 4.324 3.5629 1.518 56.0 lens body (3) Second face of −2.181 0.0000 894.498 −3.45 lens body (4) High refractive −2.181 1.0000 index layer (5) First face of 0.000 1.0000 1.5163 64.0 cover glass (6) Second face of 0.000 0.6392 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2  2.286029e+01 −4.923784e−02 8.434293e−03 3 −9.991159e−01  8.548763e−04 2.344517e−04 4 −9.971921e−01  8.783094e−04 2.339980e−04 Face a₈ a₁₀ 2 −3.738623e−02 −6.523998e−02 3  4.159776e−06  5.691857e−07 4  4.274579e−06  5.700872e−07

[0150]FIG. 13 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 2 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0151] Further, FIG. 14 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 2. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 3

[0152]FIG. 15 shows Example 3 of the present invention. In Example 3, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the first basic embodiment shown in FIG. 4. Further, it was a lens in which the F number was set to be little darker so as to improve the image quality.

[0153] The image pickup lens of Example 3 was set under the following condition: f = 3.5 mm, F No = 2.5, 2ω = 71.8, Petzval sum = 0.33 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.1000 (2) First face of 4.771 3.9000 1.518 56.0 lens body (3) Second face of −2.341 0.0000 894.498 −3.45 lens body (4) High refractive −2.341 1.0000 index layer (5) First face of 0.000 1.0000 1.5163 64.0 cover glass (6) Second face of 0.000 0.8622 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2  2.043376e+01 −3.957181e−02 5.313152e−02 3 −9.948575e−01  8.813805e−04 2.175968e−04 4 −9.939222e−01  8.903731e−04 2.172255e−04 Face a₈ a₁₀ 2 −1.398622e−01 8.238725e−02 3  6.522519e−07 6.956115e−08 4  7.494107e−07 6.502132e−08

[0154]FIG. 16 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 3 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0155] Further, FIG. 17 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 3. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 4

[0156]FIG. 18 shows Example 4 of the present invention. In Example 4, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the first basic embodiment shown in FIG. 4. Further, it was a lens in which the F number was set to be little darker and the angle of view was narrowed so as to improve the image quality.

[0157] The image pickup lens of Example 4 was set under the following condition: f = 3.7 mm, F No = 2.8, 2ω = 68.0, Petzval sum = 0.32 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.1548 (2) First face of 4.852 4.1736 1.518 56.0 lens body (3) Second face of −2.483 0.0000 894.498 −3.45 lens body (4) High refractive −2.483 1.0000 index layer (5) First face of 0.000 1.0000 1.5163 64.0 cover glass (6) Second face of 0.000 0.9523 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2  1.431475e+01 −2.907101e−02 5.371395e−02 3 −9.932373e−01  9.049265e−04 2.272390e−04 4 −9.925322e−01  9.105759e−04 2.269840e−04 Face a₈ a₁₀ 2 −1.329815e−01 9.279416e−02 3  2.835733e−06 3.444509e−07 4  2.891540e−06 3.426274e−07

[0158]FIG. 19 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 4 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0159] Further, FIG. 20 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 4. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 5

[0160]FIG. 21 shows Example 5 of the present invention. In Example 5, the second of the lens body 1 a was formed into the Fresnel face 2 a while unifying the diffraction element 3 a with the first face of the lens body 1 a as in the same manner as that of the first basic embodiment shown in FIG. 4.

[0161] The image pickup lens of Example 5 was set under the following condition: f = 3.75 mm, F No = 2.8, 2ω = 65.0, Petzval sum = 0.34 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.7055 (2) High refractive 4.592 0.0000 894.498 −3.45 index layer (3) First face of 4.593 4.3933 1.518 56.0 lens body (4) Second face of −2.522 1.0000 lens body (5) First face of 0.000 1.0000 1.5163 64.0 cover glass (6) Second face of 0.000 0.4706 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2  7.122991e+00 −2.320030e−02 1.075459e−02 3  7.130459e+00 −2.320976e−02 1.075869e−02 4 −6.780241e−01  5.885833e−03 9.060170e−04 Face a₈ a₁₀ 2 0.000000e+00 0.000000e+00 3 0.000000e+00 0.000000e+00 4 0.000000e+00 0.000000e+00

[0162]FIG. 22 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 5 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0163] Further, FIG. 23 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 5. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 6

[0164]FIG. 24 shows Example 6 of the present invention. In Example 6, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the first basic embodiment shown in FIG. 4. Further, the diffraction element 3 is also unified with the first face of the lens body 1 a.

[0165] The image pickup lens of Example 6 was set under the following condition: f = 3.7 mm, F No = 2.8, 2ω = 67.6, Petzval sum = 0.34 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.0000 (2) High refractive 4.657 0.0000 894.498 −3.45 index layer (3) First face of 4.657 4.2714 1.518 56.0 lens body (4) Second face of −2.516 0.0000 894.498 −3.45 lens body (5) High refractive −2.516 1.0000 index layer (6) First face of 0.000 1.0000 1.5163 64.0 cover glass (7) Second face of 0.000 0.8485 cover glass (8) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2  1.160906e+01 −3.077417e−02 4.610394e−02 3  1.176908e+01 −3.098202e−02 4.608045e−02 4 −1.235293e+00 −5.178718e−04 7.573794e−05 5 −1.238337e+00 −5.420760e−04 7.619796e−05 Face a₈ a₁₀ 2 −6.527001e−03 −6.823957e−03 3 −6.820853e−03 −6.630086e−03 4  2.286737e−05  3.135998e−06 5  2.300038e−05  3.119125e−06

[0166]FIG. 25 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 6 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0167] Further, FIG. 26 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 6. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 7

[0168]FIG. 27 shows Example 7 of the present invention. In Example 7, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the second basic embodiment shown in FIG. 6.

[0169] The image pickup lens of Example 7 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 68.8, Petzval sum = 0.33 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.5255 (2) First face of 2.695 3.7092 1.527 56.0 lens body (3) Second face of −1.796 0.0000 831.404 lens body (4) High refractive −1.796 0.7000 index layer (5) First face of 0.000 1.0000 1.518 64.0 cover glass (6) Second face of 0.000 0.0062 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −8.988205e+00 3.322188e−02 −7.376609e−04 3 −5.467891e−01 2.617233e−02 −7.559079e−03 4 −5.472168e−01 2.617215e−02 −7.560272e−03 Face a₈ a₁₀ 2 −3.817397e−03  5.485112e−04 3  4.930335e−03 −6.512470e−04 4  4.929550e−03 −6.511086e−04

[0170] Under such condition, d/f=1.426 was achieved, thereby satisfying the expression (1).

[0171] Further, |r₂/r₁|=0.666 was achieved, thereby satisfying the expression (2).

[0172] Furthermore, ds/f=0.202 was achieved, thereby satisfying the expression (3).

[0173]FIG. 28 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 7 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0174] Further, FIG. 29 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 7. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 8

[0175]FIG. 30 shows Example 8 of the present invention. In Example 8, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the second basic embodiment shown in FIG. 6.

[0176] The image pickup lens of Example 8 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 69.8, Petzval sum = 0.33 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.5100 (2) First face of 2.747 3.5900 1.527 56.0 lens body (3) Second face of −1.794 0.0000 831.404 lens body (4) High refractive −1.794 0.5000 index layer (5) First face of 0.000 1.0000 1.518 64.0 cover glass (6) Second face of 0.000 0.2683 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −9.171413e+00 3.435569e−02 −4.812703e−03 3 −5.488248e−01 2.604141e−02 −7.583875e−03 4 −5.492227e−01 2.603851e−02 −7.583299e−03 Face a₈ a₁₀ 2 3.170295e−04 −8.615294e−04 3 4.925911e−03 −6.523167e−04 4 4.924539e−03 −6.521111e−04

[0177] Under such condition, d/f=1.381 was achieved, thereby satisfying the expression (1).

[0178] Further, |r₂/r₁|=0.653 was achieved, thereby satisfying the expression (2).

[0179] Furthermore, ds/f=0.196 was achieved, thereby satisfying the expression (3).

[0180]FIG. 31 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 8 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0181] Further, FIG. 32 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 8. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 9

[0182]FIG. 33 shows Example 9 of the present invention. In Example 9, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the second basic embodiment shown in FIG. 6.

[0183] The image pickup lens of Example 9 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 71.2, Petzval sum = 0.32 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.5000 (2) First face of 2.814 3.4000 1.527 56.0 lens body (3) Second face of −1.792 0.0000 831.404 lens body (4) High refractive −1.791 0.5000 index layer (5) First face of 0.000 1.0000 1.518 64.0 cover glass (6) Second face of 0.000 0.3570 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −1.152514e+01 4.268985e−02 −9.093965e−03 3 −5.496560e−01 2.598847e−02 −7.595081e−03 4 −5.499979e−01 2.598139e−02 −7.591602e−03 Face a₈ a₁₀ 2 −4.017900e−04  3.479243e−04 3  4.924438e−03 −6.527990e−04 4  4.922140e−03 −6.524825e−04

[0184] Under such condition, d/f=1.308 was achieved, thereby satisfying the expression (1).

[0185] Further, |r₂/r₁|=0.637 was achieved, thereby satisfying the expression (2).

[0186] Furthermore, ds/f=0.192 was achieved, thereby satisfying the expression (3).

[0187]FIG. 34 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 9 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0188] Further, FIG. 35 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 9. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 10

[0189]FIG. 36 shows Example 10 of the present invention. In Example 10, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the second basic embodiment shown in FIG. 6. In Example 10, the lens body 1 a was made of a glass material.

[0190] The image pickup lens of Example 10 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 70.6, Petzval sum = 0.33 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.5641 (2) First face of 2.903 3.6542 1.585 59.4 lens body (3) Second face of −1.934 0.0000 831.404 lens body (4) High refractive −1.934 1.0000 index layer (5) First face of 0.0000 1.0000 1.518 64.0 cover glass (6) Second face of 0.000 −0.2670 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₈ 2 −1.053093e+01 3.653708e−02 −5.926449e−03 3 −5.487356e−01 2.603218e−02 −7.588997e−03 4 −5.491175e−01 2.602291e−02 −7.582574e−03 Face a₈ a₁₀ 2 −1.077630e−03  4.702053e−04 3  4.926600e−03 −6.525410e−04 4  4.923072e−03 −6.520399e−04

[0191] Under such condition, d/f=1.405 was achieved, thereby satisfying the expression (1).

[0192] Further, |r₂/r₁|=0.666 was achieved, thereby satisfying the expression (2).

[0193] Furthermore, ds/f=0.217 was achieved, thereby satisfying the expression (3).

[0194]FIG. 37 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 10 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0195] Further, FIG. 38 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 10. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 11

[0196]FIG. 39 shows Example 11 of the present invention. In Example 11, the second face of the lens body 1 a was formed into the Fresnel face 2 a while being unified with the diffraction element 3 a as in the same manner as that of the second basic embodiment shown in FIG. 6. In Example 11, the lens body 1 a was made of a glass material.

[0197] The image pickup lens of Example 11 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 70.6, Petzval sum = 0.32 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.5190 (2) First face of 2.852 3.5374 1.544 62.9 lens body (3) Second face of −1.802 0.0000 831.404 lens body (4) High refractive −1.802 0.5000 index layer (5) First face of 0.0000 1.0000 1.518 64.0 cover glass (6) Second face of 0.000 0.3044 cover glass (7) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −1.017267e+01 3.612915e−02 −5.889963e−03 3 −5.488893e−01 2.603868e−02 −7.586976e−03 4 −5.492270e−01 2.603057e−02 −7.581993e−03 Face a₈ a₁₀ 2 −8.053710e−04  2.435896e−04 3  4.926491e−03 −6.524366e−04 4  4.923639e−03 −6.520522e−04

[0198] Under such condition, d/f=1.361 was achieved, thereby satisfying the expression (1).

[0199] Further, |r₂/r₁|=0.632 was achieved, thereby satisfying the expression (2).

[0200] Furthermore, ds/f=0.200 was achieved, thereby satisfying the expression (3).

[0201]FIG. 40 shows the chromatic aberration of magnification of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 11 under such condition. According to the figure, it is clear that the chromatic aberration of magnification in each ray path is decreased.

[0202] Further, FIG. 41 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 11. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

[0203] Next, still another basic embodiment of the present invention will be described by referring to FIG. 42 to FIG. 44.

[0204]FIG. 42 shows still another basic stricture of an image pickup lens according to the present invention. The image pickup lens comprises a lens body 1 b made of, for example, a resin such as plastic or the like. In this basic embodiment, a second face on the image pickup surface side of the lens body 1 b is formed into an aspherical Fresnel face 2 b. It is preferable that the concave-convex shape of the aspherical Fresnel face 2 b be formed fine but not to the extent where there receives bad influence by diffraction. Also, a first face of the lens body 1 b may be formed into the Fresnel face 2 b. Furthermore, the first face of the lens body 1 b is formed into an aspherical face.

[0205] A diaphragm 3 b is disposed on the object side of the lens body 1 b, and a cover glass 4 b and an image pickup surface 5 b as a light receiving surface of an image pickup element such as a CCD, a CMOS or the like are disposed, respectively, on the second face of the lens body 1 b.

[0206] Further, in the embodiment, the lens body 1 b is to satisfy the condition represented by the following expressions:

1.17<d/f<1.4   (4)

0.58<|r ₂ /r ₁|<0.73   (5)

0≦ds/f≦0.4   (6)

[0207] where,

[0208] d denotes the thickness in the center of the lens body 1 b, f denotes the focal length of the lens body 1 b, r₁ denotes the radius of curvature in the center of the first face of the lens body 1 b, r₂ denotes the radius of curvature in the center of the second face of the lens body 1 b, and ds denotes the distance between the center of the diaphragm and the first face of the lens body 1 b.

[0209] Further, in the present embodiment, when determining the above-described expression (4) at the time of designing the lens body 1 b, first, ray tracing simulation is performed beforehand on a group of lens bodies with different d/f values under the condition where each aberration except for the distortion aberration is maintained to be excellent. Then, dependency of the distortion aberration on the d/f value is obtained by calculating the distortion aberration of the lens bodies 1 b. The d/f value as a desired distortion aberration is selected on the basis of the correlation obtained thereby so as to determine the thickness in the center of the lens body based on the selected d/f value.

[0210]FIG. 43 shows the dependency of the distortion aberration on the d/f value. In general, the amount of distortion aberration which is visually comfortable is about −4% to 2% in the field of the present invention. Thus, as shown in FIG. 43, the appropriate d/f value within the range of appropriate distortion aberration amount is set within the large frame shown in the figure. The thickness in the center of the lens body 1 b is to be determined on the basis of the d/f value within the range.

[0211] Preferably, the distortion aberration amount may fall within the range of −3% to 2%. Under the condition, the appropriate d/f value within the range of the appropriate distortion aberration amount is set within the small frame shown in FIG. 43. This can be represented by a following expression:

1.19<d/f<1.37   (3aa)

[0212] Also, in the same manner, when determining the expression (5), ray tracing simulation is performed beforehand on the lens bodies 1 b with different |r₂/r₁| values under the condition where each aberration except for the distortion aberration is maintained to be excellent. Then, dependency of the distortion aberration on the |r₂/r₁| value is obtained by calculating the distortion aberration of each lens body 1 b. The |r₂/r₁| value as a desired distortion aberration is selected on the basis of the correlation obtained thereby so as to determine the radius of curvature in the center of the lens body 1 b based on the selected |r₂/r₁| value.

[0213]FIG. 44 shows the dependence of the distortion aberration on the |r₂/r₁| value. The appropriate |r₂/r₁| value within the range of appropriate distortion aberration amount for achieving −4% to 2% distortion aberration falls within the large frame shown in FIG. 44. The radius of curvature in the center of the lens body 1 b is to be determined on the basis of the |r₂/r₁| value within the range.

[0214] Preferably, the distortion aberration amount may fall within the range of −3% to 2%. Under the condition, the appropriate |r₂/r₁| value within the range of appropriate distortion aberration amount is set within the small frame shown in FIG. 44. This can be represented by a following expression:

0.6<|r ₂ /r ₁|<0.7   (3ba)

[0215] The expression (4) is a condition for correcting the distortion aberration amount with a comfortable visual sense while well maintaining the aberration except for the distortion aberration. In the expression (4), if the d/f value is larger than 1.4, the distortion aberration becomes large in the positive direction so that the back focus becomes short. Further, the center of the lens body 1 b becomes thick so that it becomes difficult to manufacture. If the d/f value is smaller than 1.17, the distortion aberration in the negative direction becomes large. Thus, even though the distortion aberration can be suppressed, the curvature of field, the longitudinal chromatic aberration, the lateral aberration, the spherical aberration and the like cannot be well corrected.

[0216] Further, the expression (5) is a condition for achieving an ideal lens shape in which each aberration is well balanced. In the expression (5), if the |r₂/r₁| value is larger than 0.73 or smaller than 0.58, mainly the spherical aberration becomes worsen resulting in deterioration of resolution.

[0217] Furthermore, the expression (6) is a condition for correcting the coma aberration while achieving an excellent balance of coma aberration and the distortion aberration. In the expression (6), if the ds/f value is larger than 0.4, the coma aberration cannot be sufficiently corrected and if it is smaller than 0, the diaphragm is to be in the lens body 1 b. Thus, it is not desirable since it becomes difficult to manufacture.

[0218] In the basic embodiment, the second face on the image pickup surface 5 side of the lens body 1 b is formed into the Fresnel face 2 b. Therefore, by setting the Petzval sum and the curvature of field small, an excellent image plane can be obtained while decreasing each aberration, particularly, the lateral aberration and the spherical aberration. As a result, the optical characteristic can be remarkably improved.

[0219] In the present embodiment, by satisfying the expressions (4) to (6), the distortion aberration can be well corrected.

[0220] Now, Examples of the present invention will be described by referring to FIG. 45 to FIG. 59.

[0221] In the Examples, f denotes the focal length (mm) of the whole system, F No denotes F number, and 2ω denotes the maximum angle of view. Further, r denotes the radius of curvature (mm) in the center of the lens and the like, d denotes the distance (mm) between each optical surface, nd denotes the refractive index of an optical material (medium) present between with the next optical surface, and υd denotes the Abbe constant.

[0222] Provided the optical axis direction is taken as a Z-axis, the direction perpendicular to the optical axis is taken as an X-axis, and the traveling direction of light is defined to be positive, the shape of the aspheric face of the lens is represented by the above-described expression (Eq 2), where k, a₄, a₆, a₈, and a₁₀ are aspherical factors.

Example 12

[0223]FIG. 45 shows Example 12 of the present invention. In Example 12, the second face of the lens body 1 b was formed into the Fresnel face 2 b as in the image pickup lens shown in FIG. 42.

[0224] The image pickup lens of Example 12 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 68.8, Petzval sum = 0.36 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.4000 (2) First face of 2.518 3.5446 1.527 56.0 lens body (3) Second face of −1.547 0.5000 lens body (4) First face of 0.0000 1.0000 1.518 64.0 cover glass (5) Second face of 0.000 0.1779 cover glass (6) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −6.902735e+00  3.315906e−02 −3.920313e−03 3 −1.165110e+00 −5.293182e−03 1.898012e−02 Face A₈ a₁₀ 2  0.000000e+00 0.000000e+00 3 −6.143211e−03 9.680898e−04

[0225] Under such condition, d/f=1.363 was achieved, thereby satisfying the expression (4).

[0226] Further, |r₂/r₁|=0.614 was achieved, thereby satisfying the expression (5).

[0227] Furthermore, ds/f=0.154 was achieved, thereby satisfying the expression (6).

[0228]FIG. 46 shows the lateral aberration of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 12 under such condition.

[0229] Further, FIG. 47 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 12. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 13

[0230]FIG. 48 shows Example 13 of the present invention. In Example 13, the second face of the lens body 1 b was formed into the Fresnel face 2 b as in the image pickup lens shown in FIG. 42.

[0231] The image pickup lens of Example 13 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 70.0, Petzval sum = 0.35 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.3600 (2) First face of 2.553 3.3500 1.527 56.0 lens body (3) Second face of −1.619 0.5000 lens body (4) First face of 0.0000 1.0000 1.518 64.0 cover glass (5) Second face of 0.000 0.2636 cover glass (6) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −9.905958e+00  5.208922e−02 −1.378825e−02 3 −2.651238e+00 −1.631000e−02 −2.959295e−03 Face a₈ a₁₀ 2 0.000000e+00  0.000000e+00 3 4.853739e−03 −6.760713e−04

[0232] Under such condition, d/f=1.288 was achieved, thereby satisfying the expression (4).

[0233] Further, |r₂/r₁|=0.634 was achieved, thereby satisfying the expression (5).

[0234] Furthermore, ds/f=0.138 was achieved, thereby satisfying the expression (6).

[0235]FIG. 49 shows the lateral aberration of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 13 under such condition.

[0236] Further, FIG. 50 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 13. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 14

[0237]FIG. 51 shows Example 14 of the present invention. In Example 14, the second face of the lens body 1 b was formed into the Fresnel face 2 b as in the image pickup lens shown in FIG. 42.

[0238] The image pickup lens of Example 14 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 71.6, Petzval sum = 0.36 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.7191 (2) First face of 2.521 3.1344 1.527 56.0 lens body (3) Second face of −1.715 0.5000 lens body (4) First face of 0.0000 1.0000 1.518 64.0 cover glass (5) Second face of 0.000 0.3253 cover glass (6) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −8.954858e+00 4.296668e−02 −6.370016e−03 3 −7.497561e−01 1.345057e−02  9.347825e−03 Face a₈ a₁₀ 2  0.000000e+00 0.000000e+00 3 −2.552510e−03 5.660260e−04

[0239] Under such condition, d/f=1.206 was achieved, thereby satisfying the expression (4).

[0240] Further, |r₂/r₁|=0.680 was achieved, thereby satisfying the expression (5).

[0241] Furthermore, ds/f=0.277 was achieved, thereby satisfying the expression (6).

[0242]FIG. 52 shows the lateral aberration of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 14 under such condition.

[0243] Further, FIG. 53 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 14. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 15

[0244]FIG. 54 shows Example 15 of the present invention. In Example 15, the second face of the lens body 1 b was formed into the Fresnel face 2 b as in the image pickup lens shown in FIG. 42. In Example 15, the lens body 1 b was made of a glass material.

[0245] The image pickup lens of Example 15 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 70.4, Petzval sum = 0.35 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.6193 (2) First face of 2.755 3.5102 1.585 59.4 lens body (3) Second face of −1.801 0.5000 lens body (4) First face of 0.0000 1.0000 1.518 64.0 cover glass (5) Second face of 0.000 0.2181 cover glass (6) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −1.481685e+00 −4.980061e−04 3.020918e−03 3 −6.889858e−01  9.497036e−03 1.691608e−02 Face a₈ a₁₀ 2  0.000000e+00 0.000000e+00 3 −6.140451e−03 1.051427e−03

[0246] Under such condition, d/f=1.350 was achieved, thereby satisfying the expression (4).

[0247] Further, |r₂/r₁|=0.654 was achieved, thereby satisfying the expression (5).

[0248] Furthermore, ds/f=0.238 was achieved, thereby satisfying the expression (6).

[0249]FIG. 55 shows the lateral aberration of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example 15 under such condition.

[0250] Further, FIG. 56 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 15. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

Example 16

[0251]FIG. 57 shows Example 16 of the present invention. In Example 16, the second face of the lens body 1 b was formed into the Fresnel face 2 b as in the image pickup lens shown in FIG. 42. In Example 16, the lens body 1 b was made of a glass material.

[0252] The image pickup lens of Example 16 was set under the following condition: f = 2.6 mm, F No = 2.5, 2ω = 70.8, Petzval sum = 0.34 Radius r Refractive Abbe Face of Curvature Distance d Index nd constant νd (1) Diaphragm 0.000 0.4000 (2) First face of 2.659 3.4168 1.544 62.9 lens body (3) Second face of −1.656 0.5000 lens body (4) First face of 0.0000 1.0000 1.518 64.0 cover glass (5) Second face of 0.000 0.2634 cover glass (6) Image pickup 0.000 surface Aspherical Factor Face k a₄ a₆ 2 −9.602284e+00 4.139515e−02 −7.479535e−03 3 −9.689892e−01 3.136863e−03  1.172318e−02 Face a₈ a₁₀ 2  0.000000e+00 0.000000e+00 3 −3.317653e−03 5.366110e−04

[0253] Under such condition, d/f=1.314 was achieved, thereby satisfying the expression (4).

[0254] Further, |r₂/r₁|=0.623 was achieved, thereby satisfying the expression (5).

[0255] Furthermore, ds/f=0.154 was achieved, thereby satisfying the expression (6)

[0256] Further, FIG. 58 shows the lateral aberration of ray paths A, B, and C, respectively, in regards to the image pickup lens of Example. 16 under such condition.

[0257] Further, FIG. 59 shows the spherical aberration, the astigmatism, and the distortion aberration in the image pickup lens of Example 16. According to the figure, it is clear that each value of the spherical aberration, the astigmatism and the distortion aberration is almost satisfactory so that a sufficient optical characteristic can be obtained.

[0258] The present invention has been described by referring to the basic embodiments. However, it is not limited to the above-described embodiments but various modifications are possible without departing from the spirit within the scope of the present invention. 

What is claimed is:
 1. An image pickup lens comprising a lens body having a second face on an image pickup surface side of said lens body being formed into a Fresnel face and a diffraction element being unified with at least either a first face on an object side or the second face on the image pickup surface side of said lens body.
 2. An image pickup lens according to claim 1, wherein said diffraction element is unified with the second face of said lens body.
 3. An image pickup lens according to claim 1 or claim 2, wherein the first face of said lens body is formed into an aspheric face and the second face of said lens body is formed into an aspheric Fresnel face.
 4. An image pickup lens according to any one of claims 1 to 3, wherein said lens body satisfies a condition represented by a following expression: 1.25<d/f<1.45   (1) where, d: thickness in the center of said lens body; and f: focal length of said lens body.
 5. An image pickup lens according to any one of claims 1 to 4, wherein said lens body satisfies a condition represented by a following expression: 0.6<|r ₂ /r ₁|<0.75   (2) where, r₁: radius of curvature in the center of the first face of said lens body; r₂: radius of curvature in the center of the second face of said lens body.
 6. An image pickup lens according to any one of claims 1 to 5, wherein a diaphragm is positioned on the object side of said lens body and said lens body satisfies a condition represented by a following expression: 0≦ds/f≦0.4   (3) where, ds: distance between the center of said diaphragm and the first face of said lens body; and f: focal length of said lens body.
 7. A design method of an image pickup lens for determining thickness and radius of curvature in the center of a lens body having a second face on an image pickup surface side of said lens body being formed into a Fresnel face and a diffraction element being unified with at least either a first face on the object side or the second face on the image pickup surface side of said lens body, comprising the steps of: under the condition that each aberration is maintained to be excellent except for distortion aberration, performing ray tracing simulation beforehand on a lens body group with a different combination of d/f value (where, d denotes thickness in the center of a lens body and f denotes focal length of a lens body) and |r₂/r₁| value (where, r₁ denotes radius of curvature in the center of the first face of said lens body and r₂ denotes radius of curvature in the center of the second face of said lens body); calculating the distortion aberration of said lens bodies so as to obtain dependency of the distortion aberration on the d/f value and the |r₂/r₁| value; selecting, according to the correlation, the combination of the d/f value and the |r₂/r₁| value to be a desired distortion aberration; and determining the thickness and the radius of curvature of said lens body on the basis of the d/f value and the |r₂/r₁| value.
 8. An image pickup lens comprising a lens body having at least one face being formed into an aspherical face and at least either a first face on an object side or a second face on an image pickup surface side of said lens body being formed into a Fresnel face, wherein said lens body satisfies a condition represented by a following expression: 1.17<d/f<1.4   (4) where, d: thickness in the center of said lens body; and f: focal length of said lens body.
 9. An image pickup lens as claimed in claim 8, wherein said lens body satisfies a condition represented by a following expression: 0.58<|r ₂ /r ₁|<0.73   (5) where, r₁: radius of curvature in the center of the first face of said lens body; and r₂: radius of curvature in the center of the second face of said lens body.
 10. An image pickup lens according claim 8 or claim 9, wherein a diaphragm is positioned on the object side of said lens body and said lens body satisfies a condition represented by a following expression: 0≦ds/f≦0.4   (6) where, ds: distance between the center of said diaphragm and the first face of said lens body; and f: focal length of said lens body.
 11. A design method of an image pickup lens for determining thickness and radius of curvature in the center of a lens body having at least a first face on an object side or a second face on an image pickup surface side of said lens body being formed into a Fresnel face, comprising the steps of: under the condition that each aberration is maintained to be excellent except for distortion aberration, performing ray tracing simulation beforehand on a lens body group with a different combination of d/f value (where, d denotes thickness in the center of a lens body and f denotes focal length of a lens body) and |r₂/r₁| value (where, r₁ denotes radius of curvature in the center of the first face of said lens body and r₂ denotes radius of curvature in the center of the second face of said lens body); calculating the distortion aberration of said lens bodies so as to obtain dependency of the distortion aberration on the d/f value and the |r₂/r₁| value; selecting, according to the correlation of the d/f value and the |r₂/r₁| value to be a desired distortion aberration; and determining the thickness and the radius of curvature of said lens body on the basis of the d/f value and |r₂/r₁| value. 